jz47xx: Audio path tweaks:

* Increase audio buffer size to better handle IRQ latency (256->2048)
 * Ensure DMA engine is idle prior to starting transfers
 * Set AIC to repeat last sample in case of underflows

Change-Id: I9c45c20481ee072e5882b7586fb7d50bd8ef2f35

firmware/target/mips/ingenic_jz47xx/codec-jz4740.c

@@ -267,6 +267,7 @@ void audiohw_postinit(void)
 
 void audiohw_init(void)
 {
+    __aic_play_lastsample(); /* on FIFO underflow */
     i2s_codec_init();
 }
 

firmware/target/mips/ingenic_jz47xx/codec-jz4760.c

@@ -115,6 +115,7 @@ void audiohw_init(void)
     __cpm_start_aic();
 
     /* Init AIC */
+    __aic_play_lastsample(); /* on FIFO underflow.  Versus 0.. */
     __i2s_enable_sclk();
     __i2s_external_codec();
     __i2s_select_msbjustified();

firmware/target/mips/ingenic_jz47xx/pcm-jz4740.c

@@ -137,6 +137,8 @@ void DMA_CALLBACK(DMA_AIC_TX_CHANNEL)(void)
 
 void pcm_play_dma_start(const void *addr, size_t size)
 {
+    pcm_play_dma_stop();
+
     dma_enable();
 
     set_dma(addr, size);

firmware/target/mips/ingenic_jz47xx/pcm-jz4760.c

@@ -128,6 +128,8 @@ void DMA_CALLBACK(DMA_AIC_TX_CHANNEL)(void)
 
 void pcm_play_dma_start(const void *addr, size_t size)
 {
+    pcm_play_dma_stop();
+
     __dmac_channel_enable_clk(DMA_AIC_TX_CHANNEL);
 
     set_dma(addr, size);